2. Observations and data reduction
We took optical spectra of Mkn 110 at 24 epochs from February 1987 until June 1995. The sampling of the observations extends from days to years. In Table 1 we list our observing dates and the corresponding Julian Dates. The spectra were obtained at Calar Alto Observatory in Spain with the 2.2 m and 3.5 m telescopes as well as at McDonald Observatory in Texas with the 2.1 m and 2.7 m telescopes. The individual exposure times range from 10 minutes to 1 hour (see Table 1). We used spectrograph slits with projected widths of 2 to 2.5 arcsec and 2 arcmin length under typical seeing conditions of 1 to 2 arcsec. We extracted spectra of the central 5 arcsec. The slit was oriented in north-south direction in most cases.
Table 1. Log of observations
To investigate the spatial extension of the narrow line region we took spectra at different position angles: 0o, 45o, 90o, and 135o. A possible extended [OIII]5007 emission line flux was always less than 3% of the nuclear point-like emission. Mkn 110 has been inspected by Nelson et al. (1996) with the Hubble Space Telescope WFPC-1 in the near infrared spectral range. They detected a dominant unresolved nucleus in this galaxy.
Our optical spectra typically cover a wavelength range from 4000 Å to 7200 Å with a spectral resolution of 3 to 7 Å per pixel. We used different CCD detectors in the course of this monitoring program: until 1989 a RCA-chip (1024x640), in January and July 1992 a GEC-chip (1155x768), and in August 1992 a Tektronix-chip (1024x1024).
The reduction of the spectra (flat fielding, wavelength calibration, night sky subtraction, flux calibration, etc.) was done in a homogeneous way using the ESO MIDAS package.
The absolute calibration of our spectra was achieved by scaling the [OIII]5007 line of all spectra to those obtained under photometric conditions. Our absolute [OIII]5007 flux corresponds within 5% to that obtained by Peterson et al. (1998a). For a better comparison of these two data sets we will use exactly the same [OIII]5007 flux of 2.26 erg s-1 cm- 2. Furthermore, we corrected all our data for small spectral shifts and resolution differences with respect to a mean reference spectrum using an automatic scaling program of van Groningen & Wanders (1992).
The R-band image of Mkn 110 (Fig. 1) was taken with the 2.2m telescope at Calar Alto observatory on September 20, 1993 with an exposure time of 6 minutes. Again, we reduced this CCD image with the ESO MIDAS package.
In the course of our discussion we additionally will make use of archival IUE spectra of Mkn 110 taken on February 28 and 29, 1988.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999